Metallurgical process and steels manufactured by same



United States Patent O 3,088,855 METALLURGICAL PROCESS AND STEELS MANU- FACTURED BY SAME (MARTEMPERING) Elliot S. Nachtman, Park Forest, Ill., assignor to La Salle Steel Hammond, lnd., a corporation of Delaware No Drawing. Filed Oct. 22, 1956, Ser. No. 617,269 Claims. (Cl. 148-l2.4)

This invention rel-ates to a metallurgical process for producing new and improved steel products and it relates more particularly to a metallurgical process applicable to the cold finishing of steels to produce steel products having improved mechanical and physical properties.

This invention is a modification of the processes described in the copending applications Ser. No. 518,411, Ser. No. 518,412, Ser. No. 518,413 and Ser. No. 518,414, filed June 27, 1955, now Patents No. 2,767,837, No. 2,767,835, No. 2,767,836, and No. 2,767,838, respectively.

It is an object of this invention to produce and to provide a method for producing new and improved steel products having new and improved mechanical and physical properties and difierent combinations of mechanical and physical properties.

More specifically, it is an object of this invention to provide a method, adapted for use in the cold finishing of steels, which is simple and etficient in operation to produce steels having new and improved characteristics, which may be employed to produce steels having greater uniformity in properties and characteristics from heat to heat as compared to the lack of uniformity in properties of hot rolled steels of corresponding chemistry, which may be employed to expand the range of physical and mechanical properties developed in steels, and it is a related object to produce steels of the type described.

In the aforementioned copending applications, description is made of a new metallurgical process which may be employed in the cold finishing of steels to improve the physical and mechanical properties of the steel. The metallurgical process defined embodies an elevated temperature reduction step wherein, for example, the steel is advanced through a die for taking a reduction in crosssection while the steel is at temperature within the range of 200 F. to the lower critical temperature for the steel composition or while the steel is at a temperature within the range of 200 F. to ll00-l200 F. Depending upon the temperature of the steel in the elevated temperature reduction step, various physical and mechanical properties of the steel can be changed. In many instances the physical and mechanical properties of the steel are materially improved by comparision with steels of the same chemistry given an equivalent reduction as in a cold drawing operation. Thus, by controlling the temperature of the steel in the elevated temperature reduction step, by controlling the chemistry of the steel and by controlling the amount of reduction, it is possible to produce new and improved steel products having ne wand improved physical and mechanical properties and various combinations thereof.

In my copending application filed concurrently herewith and entitled Cold Finished Steels and Method for the Manufacture of Same, Scr. No. 617,264, now Patent No. 2,924,543, description is made of a modification in the elevated temperature reduction program wherein, prior to the elevated temperature reduction step, a phase 3,083,855 Patented May 7, 1963 ice change is elfected in the steel by heating the steel to austenitizing temperature and quenching the steel whereby bainite and martensite are caused to exist in the steel structure, alone and in combination. In another of my copending applications filed concurrently herewith and entitled Metallurgical Process for Cold Finishing Steels," Ser. No. 617,266, now Patent No. 2,924,544, description is made of a further modification which embodies the elevated temperature reduction concept but wherein the steel, prior to taking the elevated temperature reduction step, is heat treated at a temperature to austenitize the steel and then quenched to effect the described phase change followed by tempering the steel at an elevated temperature within the range of 200 F. to the lower critical temperature of the steel composition. In each of these processes, utilization continues to be made of the characteristics of of the steel whereby improvements are secured in physical and mechanical properties upon reduction in cross-section by working at elevated temperatures. Each of these processes lead to various changes and improvements in physical and mechanical properties of the steel over steels of equivalent chemistry in which elevated temperature reduction steps are taken without previous heat treatment to austenitize and quench or to austenitize, quench and temper.

It has been found that still diiierent and desirable steel products can be secured by a still further modification in the elevated temperature reduction concept. In accordance with the practice of this invention, the hot rolled steel is processed through a combination of steps which includes the step of martempering the hot rolled steel and then advancing the martempered steel through a die to effect the desired reduction in cross-sectional area while the steel is at a temperature within the range of 200 F. to the lower critical temperature for the steel composition and preferably while the steel is at a temperature within the range of 450 F. to 1100-1200 F. The combination of steps which includes martempering the steel and subsequent elevated temperature reduction results in the formation of steels characterized by improvements in the strength properties of the steels and hardness over steels of the corresponding chemistry which have been given an equivalent reduction at a corresponding elevated temperature but without previous martempering and over steels which have been martempered but reduced in a cold reduction step.

This cycle may be modified as follows. Steel may be subjected to martempering. The martempering may then be followed by a tempering operation. Subsequently the steel may be drawn at elevated temperatures.

Included amongst the physical and mechanical properties that are influenced by the practice described are the strength properties of the steel such as tensile strength, impact strength, yield strength, flexural strength and the like. Desirable changes are also secured in elasticity, elongation, hardness, surface roughness, machinability, proportional limits and the like. The most significant improvements developed in steels processed in the manner described are the strength properties of the steel and the hardness of the steel. The characteristics described are capable of development generally in steels of the type which are cold finished, as by the processes of drawing or extrusion. Included are those steels which are beneficially affected by the elevated temperature reduction step as described in the aforementioned copending applications. Such steels are characterized also by the ability to strain harden or harden by some mode of precipitation or other rearrangement when worked at an elevated temperature within the range of 200 F. to the lower critical temperature for the steel composition. Representative are the non-austenitic steels having a pearlitic structure in a matrix of free ferrite and the like. While steels containing carbon over a fairly wide range can be employed, as defined in the aforementioned copending applications, including low carbon steels up to high carbon steels, it is preferred to practice the concepts described with steels having a carbon content greater than 0.040 percent.

In the process embodying the features of this invention, the temperature of the steel in the elevated temperature reduction step, the chemistry of the steel, and the amount of reduction tend to influence the level of improvements that are developed and the combination of characteristics and properties that are capable of being provided in the steel product. By proper selection of temperature, chemistry and reduction, it is possible to produce steels having markedly new and different physical and mechanical properties and it becomes possible selectively to produce steels having these improvements in mechanical and physical properties along with high levels of strength properties and low stress characteristics, as will hereinafter be pointed out.

Treatment of the steels subsequent to elevated temperature reduction, as by slow cooling in air or by quenching rapidly to cool the steel in water or oil, has little effect upon the characteristics and properties developed in the steel other than the fact that rapid cooling after elevated temperature reduction tends to produce steels having lower stress values and tends to produce steels having a preponderance of compressive stresses, especially when the elevated temperature reduction step is taken while the steel is at a temperature in excess of 700 F.

As used herein, the term elevated temperature reduction (EDT) is meant to include the step of processing the steel wherein the steel is advanced through a die, such as a draw die, extrusion die or roller die to effect reduction in cross-sectional area while the steel is at a temprature within the range of 200 F. to the lower critical temperature for the steel composition, and preferably while the steel is at a temperature within the range of 450 F. to 1100-l200 F. While not equivalent from the standpoint of the process, it has been found that many of the characteristics described are capable of being developed by other methods for Working the steel to effect reduction in cross-sectional area in an elevated temperature reduction step. For example, many of the described improvements are secured in steels which have been marternpered and then reduced in cross-section by rolling while the steel is at a temperature Within the range of 200 F. to the lower critical temperature for the steel composition.

The term "martempering is employed in the same way as it is generally used in the trade. Briefly described, martempering is intended to relate to a heat treating process wherein the steel is heated to a temperature above the transformation range, that is to a temperature above 1500 F., or to a temperature for austenitizing and then quenched in a medium which is maintained in the upper portion of the temperature range of martensite formation (M or slightly above that range and it is held in the medium until the temperature throughout the steel becomes substantially uniform. The steel is then allowed to cool through the temperature range for martensite formation.

The following will illustrate the practice of this invention with 4140 steel, which may be considered as representative of the steels capable of being employed. The following is the ladle analysis of the 4140 steel in which the major ingredients, other than iron, are set forth:

Carbon .43 Manganese .88 Phosphorus .018 Sulphur .020 Silicon .26 Chromium .86 Molybdenum .18

The hot rolled bars, as received, were descaled by pickling in sulphuric acid and limed to prevent rusting. It will be understood that other means for preparing the steel for drawing may be employed. The pickled and limed bar stock was then heated in a suitable heat treating furnace to a temperature of 1550 F. for about 45 minutes. It will be understood that the time of heating may be extended to greater than 5 minutes and even up to 60 minutes and more. The austenitized steel was then quenched in a suitable medium such as a salt bath maintained at a temperature of about 605 F. for 3 minutes. This is slightly below the M point. It will be understood that the time and temperature relationship may be estimated or calculated from the tables and data available in the art which may be used as a guide since the art usually prescribes examination of representative specimens to determine when transformation is complete since each grade of steel varries in constituents and other factors.

It will be understood that the process of martempering and the determination of the time and temperature relationship for complete transformation is well known to those skilled in the art such that detailed description of the various times and temperatures which may be employed need not be set forth in greater detail.

The marternpered steel is reheated to the temperature desired for advancement of the steel in effecting the elevated temperature reduction step. A suitable compound is applied to the surface of the marternpered steel for use in advancement of the steel through the die for elevated temperature reduction. In the development of the data hereinafter set forth, reduction was effected at elevated temperature in a drawing operation wherein the 4140 steel of inch round was given a 19.9 percent reduction.

The properties developed in the steel by the process described will hereinafter be set forth in the following table. The advantages of quenching as compared to air cooling after drawing appear to reside primarily in the reduction of residual stress characteristics, especially when use is made of a temperature in the upper range, preferably above 700 F., in the elevated temperature reduction step. As a result, the data for martempering followed by elevated temperature reduction by drawing and quenching the drawn steel need not be supplied. It will be understood that similar properties, other than lower residual stress characteristics, will be secured by quenching the steel in oil or water after the elevated temperature reduction step.

In the table hereinafter set forth, comparisons are made as between the values secured in steels processed in accordance with the practice of this invention as compared to the hot rolled steel as received, the hot rolled steel which has been cold drawn to take an equivalent reduction and as compared to the hot rolled steel which was reduced by drawing at elevated temperature but without previous heat treating to martemper the steel.

Table I {4140 steel martempered at 1550 F. and quenched to 605 F. for 180 seconds. Drawn to take a 19.9 percent reduction-air cooled after drawing] Hardness DPH Pun Tensile Yield Eh; E Temp. 01' Draw, F. Lbs, Strength, Strength, P5P

cent cent 8 MR Hot R011 140,000 105,750 15.0 42.8 510 301 301 Cold Drawn 18,320 150. 000 153,000 9.0 48.9 324 330 324 ETD 15,171 195,000 191,250 10.0 35.7 399 402 402 As quenched (0) 292,000 240,000 12.0 44.1 000 429 45s Martcmpcrcd... 214, 000 129, E00 6. 4 14. 5 458 479 479 740 11,175 186,500 135,000 12.9 52.1 425 ass 352 1,000 12, 595 145,000 132,000 21.4 59.5 335 301 29s 1 Data. at approximate peak of ETD curve. 1 conventionally quenched in oil to room temperature. This cycle may be modified by introduction of a temcooling the steel through the martensite formation range pering step between the martempering and elevated temto form martensite and then advancing the martempered perature drawing. Such a cycle is described in Table martensitic steel through a die to effect a reduction in H, the procedure being exactly the same as described for Table I except that the martempered steel was subsequently martempered at 600 F. and then drawn at several elevated temperatures.

Table II cross-sectional area while the steel is at a temperature within the range of 200 F. to the lower critical temperature for the steel composition.

2. A steel having new and improved physical and reduction-air cooled after drawing] Hardness, DPH Pun Tensile Yield gg g 2 Temp. otDraw,F. Strength, Strength,

D'SL cent cent S MR 0 Hot Roll 140,000 105, 750 15.0 42.8 310 30? 301 Cold Drawn. 10,320 150,000 153,000 9.0 43.9 324 530 3224 EDT 15,171 195,000 101,250 10.0 95.7 389 402 402 AsQuenched 292,000 240, 000 12.9 44.1 000 429 s Martempered. 214,000 129, 0.4 14.5 455 479 479 Mnrternpered, tempered at:

1 Data at approximate peak of ETD durve.

1 conventionally quenched in oil to room temperaturc.

From the results given, it will be apparent that immechanical properties produced by the method of provements are secured in the strength properties of the 45 claim 1. steel and in hardness when the steels are processed in 3. The metallurgical process of treating steel which accordance with the practice of this invention by comstrain hardens and which hardens by some mode of preparison with elevated temperature reduction alone and cipitation when worked at a temperature within the range over cold drawn steels, with or without previously marof 200 F. to the lower critical temperature for the steel tempering. 5o composition comprising the combination of steps of It will be evident that by the combination of steps martempering the martempered martensitic steel by heatcomprising martempering and elevated temperature reing the steel about its transformation range, quenching duction improvements can be introduced into steel prodthe steel in a medium maintained at a temperature in ucts thereby to provide a new and improved steel having the upper portion of the range for martensite formation, improved characteristics and different combinations of holding the steel at about said temperature for temperacharacteristics by comparison with steels of the type hereture equalization, and then cooling the steel through tofore capable of being produced by various processing the martensite formation range to form martensite and techniques. advancing the steel through a draw die to effect a reduc- It will be understood that the invention may be praciion in cross-sectional area by a drawing operation while ticed on steel rods, wires, tubing and the like, as well as the steel is at a temperature w1th1n the range of 200 on bar stock and that changes may be made with respect to the lower cr1t1cal temperature for the steel composito the techniques of heating and cooling the steel and in their method of handling through the various process metalhlrglca} Process of treating Steel w i steps without departing from the spirit of the invention, f f t b and i g hirdens by some FhQ Pi especially as defined in the following claims. up] a mu w wot e a terpil'erature wit m t e I Claim range of 200 F. to the lower cr1t1cal temperature for the steel composition comprising the combination of steps The i f fi process R trteatlrzg steel if of martempering the martempered martensitic steel by non'austemtic type f i a f 1 s me a a I f heating the steel about its transformation range, quenchof free m compnsmg t e P manor 0 Steps ing the steel in a medium maintained at a temperature martemperlng the steel by heating the steel ab its in the upper portion of the range for martensite formatTaY 1$f0fmatln range: quench mg the steel in medium tion, holding the steel at about said temperature for malfltflmed a! a temperatur? the PPP P 05 the temperature equalization, and then cooling the steel range for martensite formation, holding the steel at about through th t it f m ti range to f m martensaid temperature for temperature equalization, and then site and advancing the steel through an extrusion die to effect a reduction in cross-sectional area by an extrusion operation while the steel is at a temperature within the range of 200 F. to the lower critical temperature for the steel composition.

5. The metallurgical process of treating steel which strain hardens and which hardens by some mode of precipitation when worked at a temperature Within the range of 200 F. to the lower critical temperature for the steel composition comprising the combination of steps of martempering the martempered martensitic steel by heating the steel about its transformation range, quenching the steel in a medium maintained at a temperature in the upper portion of the range for martensite formation, holding the steel at about said temperature for temperature equalization, and then cooling the steel through the martensite formation range to form martensite and rolling the steel to effect a reduction in cross-sectional area by a rolling operation while the steel is at a temperature within the range of 200 F. to the lower critical temperature for the steel composition.

6. The metallurgical process of treating steel which strain hardens and which hardens by some mode of precipitation when worked at a temperature within the range of 200 F. to the lower critical temperature for the steel composition comprising the combination of steps of martempering the martempered martensitic steel by heating the steel about its transformation range, quenching the steel in a medium maintained at a temperature in the upper portion of the range for martensite formation, holding the steel at about said temperature for temperature equalization, and then cooling the steel through the martensite formation range to form martensite and advancing the steel through a die to effect a reduction in cross-sectional area While the steel is at a temperature within the range of 200 F. to the lower critical temperature for the steel composition, and air cooling the steel after the elevated temperature reduction step.

7. The metallurgical process of treating steel which strain hardens and which hardens by some mode of precipitation when worked at a temperature within the range of 200 F. to the lower critical temperature for the steel composition comprising the combination of steps of martempering the martempered martensitic steel by heating the steel about its transformation range, quenching the steel in a medium maintained at a temperature in the upper portion of the range for martensite formation, holding the steel at about said temperature for temperature equalization, and then cooling the steel through the martensite formation range to form martensite and advancing the steel through a die to effect a reduction in cross-sectional area while the steel is at a temperature within the range of 200 F. to the lower critical temperature for the steel composition, and quenching the steel rapidly to cool the steel after the elevated temperature reduction step.

8. The metallurgical process of treating steel which strain hardens and which hardens by some mode of precipitation when worked at a temperature within the range of 450 F. to the lower critical temperature for the steel composition comprising the combination of steps of martempering the steel by heating the steel about its transformation range, quenching the steel in a medium maintained at a temperature in the upper portion of the range for martensite formation, holding the steel at about said temperature for temperature equalization, and then cooling the steel through the martensite formation range to form martensite and advancing the martempered martensitic steel through a die to effect a reduction in cross-sectional area while the steel is at a temperature within the range of 450 F. to the lower critical temperature for the steel composition.

9. The metallurgical process of treating steel which strain hardens and which hardens by some mode of precipitation when worked at a temperature within the range of 200 F. to the lower critical temperature for the steel composition comprising the combination of steps of martempering the martempered martensitic steel by heating the steel about its transformation range, quenching the steel in a medium maintained at a temperature in in the upper portion of the range for martensite formation, holding the steel at about said temperature for temperature equalization, and then cooling the steel through the martensite formation range to form martensite and advancing the steel through a die to effect a reduction in cross-sectional area while the steel is at a temperature within the range of 200 F. to the lower critical temperature for the steel composition.

10. The method as claimed in claim 9 which includes the additional step of tempering the martempered martensitic steel prior to the elevated temperature reduction step.

References Cited in the file of this patent UNITED STATES PATENTS Harvey Sept. 13, 1955 OTHER REFERENCES 

1. THE METALLURAGICAL PROCCESS OF TREATING STEEL OF THE NON-AUSTENITIC TYPE HAVING A PEARLITIC STRUCTURE IN A MATRIX OF FREE FERRITE COMPRISING THE COMBINATTION OF STEPS OF MARTEMPERING THE STEEL BY HEATING THE STEEL ABOVE ITS TRANSFORMATION RANGE, QUENCHING THE STEEL IN A MEDIUM MAINTAINED AT A TEMPERATURE IN THE UPPER PORTION OF THE RANGE FOR MARTENSITE FORMATION, HOLDING THE STEEL AT ABOUT SAID TEMPERATTURE FOR TEMPERATURE EQUALIZATION, AND THEN COOLING THE STEEL THROUGH THE MARTENSITTE FORMATTION RANGE TO FORM MARTTENSITE AND THEN ADVANCING THE MARTEMPERED MARTENSITIC STEEL THROUGH A DIE TO EFFECT A REDUCTION IN CROSS-SECTIONAL AREA WHILE THE STEEL IS AT A TEMPERATURE WITHIN THE RANGE OF 200$ F, TO THE LOWER CRITICAL TEMPERATURE FOR THE STEEL COMPOSITION. 